Within the past 25 years the number of chemicals introduced into the environment has increased dramatically. Surprisingly, there is relatively little known about the toxicologic and/or carcinogenic potential of many of them, even though their use may be widespread and the potential for human exposure is relatively high. The National Toxicology Program is attempting to fill this gap in knowledge through the sponsorship of well-defined bioassay evaluations on a number of these materials. As part of this program, the studies conducted under this contract were designed to establish basic toxicity data on p-Nitroaniline and o-Nitroanisole, using Fisher 344 rats and B6C3F1 mice. Then, based on the data obtained, additional testing in the form of long-term toxicologic/carcinogenic evaluations are pursued in order to provide sufficient data to establish the relative safety or hazard to humans.